Disclosed herein is a process for preparing a pigment dispersion.
Pigment dispersions for pigmented phase change ink (sometimes referred to as hot melt ink or solid ink) require milling of pigments to a particle size preferably as small as the primary particle size of the pigment which can be 100 nanometers in diameter or less. Pigment size reduction can be costly and energy intensive. Known technologies that are suitable for dispersing pigments in wax media at temperatures higher than 100° C. include attrition, extrusion and homogenization. These processes require excessive milling times (up to 7 days milling time) and in some cases known processes are not even capable of delivering the required pigment particle size. Furthermore, in processes that utilize milling media (stainless steel, silica, zirconia, etc.) media wear can result in ink contamination.
Pigment dispersion is an important step in the manufacturing of pigmented phase change ink. Pigments are often supplied by the manufacturer as dry aggregates and agglomerates many times larger than the primary particle size (usually on the order of less than about 100 nanometers in diameter). These aggregates or agglomerates have to be reduced in size to less than about 100 nanometers in diameter with a narrow particle size distribution to form a stable dispersion in the solid ink medium. This is needed for the resulting solid ink formulation to demonstrate good jetting and print quality performance.
Processes for particle dispersion and pigment size reduction, such as milling and homogenization, are known. These known processes operate at room temperature, or at elevated temperatures, but still significantly below 100° C.
A need remains for a process for dispersion of pigments for pigmented phase change ink wherein the pigment must be dispersed at a high temperature, such as in a wax medium at a temperature of about 120° C. or higher, wherein the said high temperature is above the melting point of said wax medium. High temperatures present serious challenges for most milling technologies. For example, if there are regions in the mill where the temperature can drop below the melting point of the wax, especially during startup, solidification of the wax can occur causing stoppage. Most milling technologies, therefore, are not suitable for waxes at such high temperatures. Indeed, many mill manufacturers have virtually no experience running their equipment at these temperatures.
High temperature processes that can be used for dispersing pigments for phase change ink applications include extrusion, homogenization and/or attrition. These processes have been examined, but they are lengthy in process time (taking many hours or days) resulting in high cost, and in many cases do not reduce the pigment particle size to the desired size. Due to the length of the processes, contamination of the pigment dispersion by media wear (erosion) is also a concern.
Currently available pigment dispersion processes are suitable for their intended purposes. However a need remains for improved pigment dispersion processes. Further, a need remains for an improved pigment dispersion process suitable for dispersing pigments in phase change ink employing high melting wax carriers. Further, a need remains for an improved pigment concentrate that can be used for dispersion processes for solid inks. Further a need remains for improved pigment concentrates for pigment dispersion processes wherein an intermediate highly concentrated pigment concentrate can be prepared and then let down to its final concentration in a final product such as a phase change ink.
The appropriate components and process aspects of the each of the foregoing U. S. patents and Patent Publications may be selected for the present disclosure in embodiments thereof. Further, throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.